1. Field of the Invention
The present invention relates to a coating apparatus and a coating method, and in particular, to a coating apparatus and a coating method which, at the time of the start of coating, can reliably coat a coating liquid onto an object to be coated such as an aluminum web or the like.
2. Description of the Related Art
Currently, the process of forming an oxidation protective layer from a thin film of an oxygen non-permeable resin, such as polyvinyl alcohol or the like, on the surface of the photosensitive layer of an original lithographic printing plate, so as to protect the photosensitive layer from oxygen in the air, is widely carried out.
In forming the oxidation protective layer, usually, a slide bead type coating apparatus equipped with a slide bead coater and a back-up roller is generally used. A discharge slit, which discharges a coating liquid such as a solution of an oxygen non-permeable resin, and a slide surface, along which the coating liquid discharged from the discharge slit flows down, are formed at the slide bead coater. The back-up roller is provided in a vicinity of the distal end of the slide surface at the slide bead coater, and conveys, in a fixed direction, an original lithographic printing plate which is trained therearound. In this slide bead type coating apparatus, while the original lithographic printing plate is conveyed by the back-up roller such that the photosensitive layer is facing outwardly, the solution is discharged from the discharge slit, and flows down along the slide surface. Between the distal end portion of the slide surface and the surface of the photosensitive layer of the original lithographic printing plate, the solution forms a bridge of coating liquid (a coating bead) such that the solution is coated.
In the slide bead coating apparatus, at the time of the start of coating, the bridge of coating liquid must be reliably formed between the distal end of the slide bead coater and the object to be coated such as the original lithographic printing plate or the like.
Conventionally, the slide bead coater is made to approach the back-up roller such that the clearance between the distal end of the slide bead coater and the original lithographic printing plate on the back-up roller is narrowed. A bridge of the coating liquid is formed between the distal end of the slide bead coater and the lithographic printing plate, and coating of the coating liquid begins.
However, at the time of the start of coating, the coating liquid which has been discharged from the slide bead coater may excessively adhere to the object to be coated, and the coating thickness of the coating liquid at the portion of the object to be coated where coating begins may be thicker than at other portions, such that a thickly coated portion is formed. When a thickly coated portion is formed, undried portions may remain, and the coated film at the thickly coated portion may be too thick.
Further, in a case in which the width of the slide bead coater is greater than the width of the object to be coated, when coating starts by the above-described method, the coating liquid may adhere to portions of the back-up roller at the outer sides of the original lithographic printing plate. Namely, there are cases in which the coating liquid is transferred, and the transferred coating liquid flows around to the reverse surface of the original lithographic printing plate, such that the reverse surface of the original lithographic printing plate is dirtied.